1. Field of the Invention
The present invention relates generally to flexible couplings for rotary shafts, more particularly to rotary shaft couplings which accommodate misaligned drive axes with radial movement of a transverse cylindrical torque transmitting engagement member, and most specifically to rotary shaft couplings which accommodate misaligned axes with radial movement of a transverse cylindrical torque transmitting engagement member primarily produced by molding.
2. General Background
Oldham type rotary shaft couplings that use a radially moveable torque transmitting engagement member are well known in the art. Modification of the Oldham principle by using a substantially cylindrical engagement member is also known. By using mating male and female members which respectively present a transverse projection and a recess that engage over half of a cylindrical form the coupling can accommodate large parallel, i.e. offset, misalignment of axes while also providing against the axial disconnection inherent to an Oldham type rotary shaft coupling. Given two pairs of perpendicularly opposed male and female mating substantially cylindrical engagement members with sufficient spacing between the three bodies large angular, in addition to large offset, misalignments are readily accommodated.
The male projection must, however, possess a convex substantially cylindrical exterior surface which is further undercut in connection to the body concerned. This is considered a particularly difficult configuration to either machine or mold. The female cavity can easily be effected with an appropriate transverse drilling of the body concerned but replicating the opposite form by machining requires rotation of the body relative to the cutting tool during cutting repeatedly along the entire length of the cylindrical projection desired as the radius of the cutting tool is restricted by the relatively small radius of the cylinder approximated in comparison with the length required of this form.
The undercut required to present the majority of a cylindrical form, moreover, presents problems to molding the male engagement member. Molding of the female engagement member is considered less problematic if no additional undercut is incorporated. The substantially cylindrical cavity must be effected by a machined substantially cylindrical projection and any additional undercut inhibits release of the molded part from this projection.
An early example of a rotary shaft coupling using substantially cylindrical male and female engagement members is found in U.S. Pat. No. 1,298,680 for a xe2x80x98Flexible Couplingxe2x80x99 issued Apr. 1, 1919 to J. R. Dunham. The female members in this coupling have two perpendicularly disposed substantially cylindrical grooves (5) which intersect. One male member or knuckle rib (4) is also equipped with a similar cylindrical groove (4xe2x80x2) to accommodate the opposed knuckle rib (9). The female member is hence essentially a cylindrical body with transverse perpendicular grooves (5) open to opposed faces of the body pivotably encasing the two opposed, engaged, knuckle ribs. Each terminal end has a knuckle rib (4) with a transverse substantially cylindrical groove (4xe2x80x2) encased by a female body member. A medial male unit (7) has two opposed knuckle ribs and five components comprise the basic structure of the coupling about which a simple cylindrical shell is disposed to encase the coupling, retain lubrication used thereupon, and limit excessive lateral movement.
A simplification, in the number of components, of this same basic approach is found in U.S. Pat. No. 2,251,126 for a xe2x80x98Molded Composition Slipper Bearingxe2x80x99 issued July 29, 1941 to T. L. Gatke that uses only three components: a female coupling end with a transverse substantially cylindrical groove, a male coupling end with a uniform projection having a transverse cylindrical aperture open to either of two parallel faces, and the slipper bearing which fits in the cylindrical groove of the female coupling end and has a reduced diameter transversely disposed cylindrical pivot through the aperture through the male projection. The slipper bearing is pivotably held in a cylindrical transverse groove of the female coupling end by opposed heads that are semicircular in the plane normal the transverse axis of the groove and transversely elongated parallel that groove to fit the same.
Most significantly, however, the slipper bearing, which possesses a rather complex configuration with two partly and one full cylindrical exterior surfaces, is molded of composition material. Woven fabric, which can be reinforced with wire, provides a core which is laminated with a non-metallic material, i.e. resin, impregnated therein bonding the fabric layers together and providing a hard xe2x80x98envelopexe2x80x99 suited to a bearing surface. The laminated fabric core provides resilient strength for impact resistance. The heads of the slipper bearing are specifically made in this manner and are apparently of one piece with the reduced diameter pivot shaft between the two heads which is further apparently enclosed by the engaging male member. How this construction is achieved is not explained however.
U.S. Pat. No. 2,260,567 issued Oct. 28, 1941 to the same inventor and possessing the same title, though resulting from a separate and earlier application, addresses this practical difficulty by making the slipper bearing from several different components, two heads and the connecting shaft, held together with a nut and bolt and other hardware including washers, bushings, and two hubs between the heads and the cylindrical shaft compressed therebetween. This construction enables practical use of a male member having a through hole open to either of two opposed, flat, parallel faces as the shaft of the bearing can be disposed in this through hole and the heads attached to either end of the shaft with the nut and bolt. The head of the bolt and the nut are each disposed in an open cavity of one head and practical resolution of this xe2x80x98simplificationxe2x80x99 is seen to involve over a dozen separate elements assembled together to provide this one component.
U.S. Pat. No. 4,941,861 issued Jul. 17, 1990 to Robert A. Painter for an xe2x80x98Interlocking Shaft Couplingxe2x80x99 discloses use of only three molded plastic components engaging each other with two pairs of substantially cylindrical transverse male and female elements. While the medial xe2x80x98disk shaped bodyxe2x80x99 is shown (FIG. 2) in an alternative arrangement having two transverse perpendicular substantially cylindrical grooves each open to an opposed face, no mating male elements are shown, and the only full depiction of the coupling utilizes male elements upon the medial component and one mating female element upon each terminal component. It is also noted that the grooves depicted in this alternative medial xe2x80x98disk shaped bodyxe2x80x99 clearly both possess a depth exceeding half the width of this body and must therefore intersect and prevent engagement by the two male elements necessary for torque transmittal.
This intersection is not depicted though a central hole is depicted in one (FIG. 3) of the versions of this medial xe2x80x98disk shaped bodyxe2x80x99 with two perpendicular transverse substantially cylindrical male elements which is explained, in the most specific reference found to the molding preferred, as being desirable because xe2x80x9c(w)hen plastic is molded, areas with different cross-sectional thicknesses typically cure at different ratesxe2x80x9d and the overlap of male elements would, absent the xe2x80x98central holexe2x80x99 be xe2x80x9cabout one and one-half times as thick as any other partxe2x80x9d by which it is understood that the thickness of the medial xe2x80x98disk shaped bodyxe2x80x99, as depicted in every instance, is approximately equal in thickness to the height of the male element or the depth of the female element. Secondly, this xe2x80x98holexe2x80x99 serves as a recess for a xe2x80x98central protrusionxe2x80x99 upward from the bottom of each transverse substantially cylindrical groove required to prevent lateral displacement and disengagement of the coupling (FIGS. 4A and 4C, Column 6, Lines 17-59). The use of flats and set screws for securing the shaft in the bore of either terminal component is also clearly shown and described by Painter.
U.S. Pat. No. 5,421,780 issued to Ivan Vukovic Jun. 6, 1995 for a xe2x80x98Joint Assembly Permitting Limited Transverse Component Displacementxe2x80x99 discloses a similar basic arrangement of the same three basic components: two terminal ends each having one transverse substantially cylindrical groove and one medial disk shaped component having two opposed perpendicular transverse substantially cylindrical keys mating the grooves and transmitting torque. Rather than using a xe2x80x98central protrusionxe2x80x99 of the groove fitting in a xe2x80x98central holexe2x80x99 of the medial disk to retard lateral displacement Vukovic utilizes two springs each axially disposed with respect to one cylindrical key adjustably biased against the same with a set screw at one end and protruding from the other against a sleeve about the cylindrical body of each terminal component.
This chronological review of the prior art considered most pertinent to the present invention suggests two basic areas of difficulty in development of rotary shaft couplings utilizing substantially cylindrical transverse engagement elements: undesired lateral displacement of the engagement members with respect to each other; and a satisfactory method of manufacture.
The earliest references cited relied upon an expensive machined construction of five basic components using four male engagement members engaging each other within two female body components and an exterior sleeve retaining lubrication and limiting excessive lateral displacement. Machining the exterior cylindrical surfaces of the transverse male engagement members, i.e. xe2x80x98rib knucklesxe2x80x99 in this reference, is considered particularly expensive.
The first known use of molded components, which are generally considered to be much less expensive than machined components, apparently did not have a concern with lateral displacement but did have a large concern with practical manufacture and the three piece construction recognized as being the simplest possible use of two perpendicular transverse substantially cylindrical engagements was found to be problematic with the solution requiring a large number of pieces of hardware in addition to the division of one of the three main components into many subsidiary pieces.
The first known rotary shaft coupling utilizing only three main components and two perpendicular transverse substantially cylindrical engagements relied upon a central protrusion of each groove engaging a central hole through the medial component having both male engagement members extending from opposed faces to prevent lateral displacement of the engaging members. This aspect is given detailed treatment in the last reference cited with use of springs adjustably biased against tight fitting sleeves.
The earlier references are noted to differ from the last two in essentially having two male members directly engaged with each other and additionally held, as a double pivot, within a female cavity in contrast to three main components: two terminal and one medial. In both of the later cases the medial component has two opposed male members, each terminal component presents one female engagement, and provision is made for preventing lateral displacement of the engaging members. No viable alternative arrangement in which the medial component has both female engagements is disclosed. Specifics with regard to manufacture by molding are restricted to the observation that widely varying cross sections are to be avoided as causing warping.
The earlier use of molding referenced above used fabric lamination in what is presumed to be a resin molding. This type of molding and material, i.e. thermoset casting, is recognized as generally predating more modern injection molding of thermoplastics which is assumed to be the method utilized for the latter reference specifying construction in plastic as the implied manufacture requires molding under pressure which aspect characterizes injection molding.
Casting is not pressurized, relies upon gravity to fill the mold, and is hence distinguished from injection molding. Both are types of molding and may use a fully enclosed mold but casting can also use an open mold. Casting is unsuited to obtaining complex configurations because of the gravity feed and to molding thermoplastics generally owing to insufficient density of the material. Thermoset plastics are commonly cast with a resin which undergoes a catalyst induced reaction in xe2x80x98settingxe2x80x99 and does not require the application of heat. Injection molding, in contrast, involves melting plastic chips behind the barrel of an injection machine and forcing the melt under considerable pressure into the necessarily closed and preferably two part mold. It is also preferred that a two part mold be used in casting because two parts, as opposed to three, four or more parts, can be readily cooled, disassembled, and reassembled automatically.
Molding is also considered the most economic means known for manufacturing these components but certain problems inherent to manufacture of the three basic components of a rotary shaft coupling utilizing perpendicular transverse substantially cylindrical engagements by this means are recognized. Use of a medial component presenting both female engagements is suspected of difficulty because the idea was put forth in an infeasible manner. If the component is injection molded both the shape and the depth of the two perpendicular transverse substantially cylindrical cavities open to either of the two opposed faces are recognized as imposing severe difficulties. The two female engagements cannot overlap or the engaging male members on the terminal components cannot both fully engage the medial component. And the two cannot easily avoid overlapping each other without increasing the thickness of the medial component well past the point of being xe2x80x98disk shapedxe2x80x99 which is considered in the prior art to be necessary.
To avoid warping consequent to cooling of widely varying cross sections a medial component presenting both female engagements would need to be over three times the depth of each substantially cylindrical cavity to avoid having most of the component being over xe2x80x98one and one half times as thickxe2x80x99 as the portion remaining behind either of the two substantially cylindrical cavities is otherwise necessarily of lesser thickness than the cavity. This would result in a much thicker medial component in accordance with the prior art that is considered infeasible.
The shape of the two opposed transverse substantially cylindrical cavities required of the coupling also presents a large difficulty to manufacture by injection molding. The depths of the undercuts on either side of the cavity must be minimized, as clearly seen to be the case in U.S. Pat. No. 4,941,861, in order to use an axially aligned two part mold. This enables each part of the mold having the positive form of the cylindrical member to be removed axially from the cooled part.
This principle also holds in molding of the male members in an conventional axially aligned two part mold as again clearly seen to be the case in both the male and female engagement members in U.S. Pat. No. 4,941,861. There is no additional undercut to the substantially cylindrical form in either case as the cylindrical form simply intersects the proximate flat surface. This minimizes the variation in cross sectional thickness of the molding as does use of male rather than female opposed engagements required of the medial component which aspects are both hence seen to be inherent to a rotary shaft coupling using these three basic components each manufactured by conventional injection molding using axially aligned two part molds.
Minimization of the undercut of both positive substantially cylindrical forms required of conventional injection molding with an axially aligned two part mold, however, results in substantially flush opposed faces in engagement as clearly seen in U.S. Pat. No. 4,941,861 which effectively eliminates the spacing between these surfaces required of a fundamental action desired of this type of coupling: pivoting about each axis of substantially cylindrical engagement. Each engagement is effectively restricted to lateral displacement and while this accommodates lateral, i.e. offset, misalignment it does not accommodate angular misalignment. It is also considered that restriction to lateral displacement exacerbates that motion thereby necessitating additional features to restrict the same.
Because it is desirable to mold, preferably with a two part mold, each of the three basic components of a rotary shaft coupling utilizing perpendicular transverse substantially cylindrical engagements and obtain a coupling which is relatively unrestricted in accommodating both angular and parallel misalignment it is considered that a need exists for such a coupling which further preferably does not require additional features to prevent undesired lateral displacement and which permits use of a medial component presenting both female engagements having a thickness of less than three times the depth of the substantially cylindrical engagements provided.
The encompassing object of the present invention is to provide a rotary shaft coupling having three basic components utilizing perpendicular transverse substantially cylindrical engagements which is relatively unrestricted in accommodating both angular and parallel misalignment.
A primary auxiliary object of the present invention is to mold three basic components of a rotary shaft coupling utilizing perpendicular transverse substantially cylindrical engagements which is relatively unrestricted in accommodating both angular and parallel misalignment.
A secondary auxiliary object of the present invention is to utilize two part molds in molding three basic components of a rotary shaft coupling utilizing perpendicular transverse substantially cylindrical engagements which is relatively unrestricted in accommodating both angular and parallel misalignment.
A primary ancillary object of the present invention is to provide a rotary shaft coupling utilizing perpendicular transverse substantially cylindrical engagements which is relatively unrestricted in accommodating both angular and parallel misalignment having two terminal components and one medial component presenting both substantially cylindrical transverse cavities perpendicularly disposed on opposed faces.
A secondary ancillary object of the present invention is to provide a rotary shaft coupling utilizing perpendicular transverse substantially cylindrical engagements which is relatively unrestricted in accommodating both angular and parallel misalignment having two terminal components and one medial component presenting both substantially cylindrical transverse cavities perpendicularly disposed on opposed faces possessing a thickness less than thrice the depth of each said cavity.
Another secondary ancillary object of the present invention is to provide a rotary shaft coupling utilizing perpendicular transverse substantially cylindrical engagements which is relatively unrestricted in accommodating both angular and parallel misalignment which does not have additional features to retard lateral displacement of each engagement.
In achievement of the above stated objectives a rotary shaft coupling comprised of one medial component having two opposed perpendicular transverse substantially cylindrical cavities, hereinafter known for the sake of brevity as transverse cavities, and two terminal components each having one transverse substantially cylindrical extension, hereinafter known for the sake of brevity as a transverse extension, spaced apart from the adjacent face by a reduced section transverse foot is suggested. With each terminal component molded with a part line parallel the shaft axis bisecting the transverse extension, including the reduced section transverse foot, use a two part mold is enabled. Spacing of the transverse extension apart from the adjacent face of each terminal component also facilitates minimization of the width of the medial component and molding of the same with a two part axially aligned mold with a part line about the equator of a substantially cylindrical body having a thickness of only slightly more than twice the depth of either transverse cavity.
It is further specifically suggested that the medial component with the two perpendicularly opposed transverse cavities be molded by injection molding preferably using a two part mold with part line effected upon the equator of the substantially cylindrical body as suggested above. Use of a relatively dense, high quality, thermoplastic is also recommended. Use of relatively large diameter transverse cavities with respect to the body diameter, no additional undercutting with respect to the adjacent surface, and minimal body between the central crossing of these relatively large transverse cavities results in a generally waved configuration of sufficiently uniform cross section for accurate injection molding with a simple two part axially aligned mold.
For each terminal component it is also further specifically suggested that a substantially cylindrical body, with one transverse extension and reduced section foot spacing the same apart from the adjacent interior face and a bore for a shaft end on the opposed exterior face, be molded in metal. It is preferred that die casting be utilized, specifically die cast zinc with a nickel plating, but other materials and other types of molding can be utilized with economic molding of this component ensured by use of a two part mold with the part line bisecting the transverse extension and foot.
Use of an injection molded thermoplastic medial component with transverse cavities having no additional undercut together with terminal components each die cast in metal and possessing a transverse extension spaced apart from the adjacent interior surface by a reduced section foot also enables both lateral sliding and axial compression assembly and disassembly. In addition, favorable surface engagement characteristics are readily obtained in opposing the resilient plastic body presenting the transverse cavities to the rigid metal transverse extension. Injection molding and die casting both possess superior accuracy in production which can ensure a very close compressed fit of the transverse extension within each transverse cavity by the body of the medial component.
The essential configuration inclusive of the transverse extension spaced apart from the adjacent interior surface of each terminal component and two perpendicular transverse cavities presented by opposed faces of the medial component without additional offset accommodates both large parallel and angular shaft misalignments. Parallel misalignments of ten percent of the basic coupling diameter are readily accommodated as are angular misalignments of five degrees at shaft speeds up to 3,000 revolutions per minute (rpm). A coupling in accordance with the principles relating to the present invention is also readily utilized for transmission of reciprocating force rather than torque. Further advantages may be appreciated with reading of the detailed discussion below; especially with reference to the drawings appended hereto and briefly discussed directly below.